Various implants of this type are already known, and conventionally they comprise two elements for bearing against the overlying and underlying vertebral plates, and means for verifying the distance between said bearing elements so as to match the implant to the height required as a function of the morphology of the patient""s spine.
The means for varying the height of the implant can comprise a rotary member, together with screw-and-nut type means.
A major drawback of that known implant lies in that the rotary member must be turned about an axis that is substantially parallel to the axis of the spinal column. This can make it extremely difficult to bring the implant to the desired height since the tool used for turning said member, given the surrounding environment of the patient, can be turned through only a small fraction of a complete turn, so it must be maneuvered many times in order to reach the desired height. This is made worse by the fact that the thread must necessarily be fine so as to generate the axial force required for separation purposes without running the risk of producing slip between the bearing elements and the over-and underlying vertebral plates.
The present invention seeks mainly to mitigate that drawback, and to propose an implant for replacing a vertebral body, which implant is simpler to handle when being put into place when performing separation, and which is also provided with excellent stability against the axial compression forces to which it will be exposed.
Thus, the present invention provides an implant specifically for replacing a vertebral body in surgery of the spine, the implant comprising first and second bearing elements for bearing against under-and overlying vertebral plates, retaining means for retaining the two bearing elements one above the other, and at least one moving member suitable for varying the distance between said bearing elements, the implant being characterized in that the retaining means comprise at least one slideway means provided between the two bearing elements, and in that the or each moving member is constituted by a cam having discrete positions suitable for being rotated about an axis that is essentially horizontal and essentially parallel to the sagittal plane.
Preferred but non-limiting features of the implant of the invention are as follows:
the or each discrete-position cam has, in cross-section, an outline in the form of an irregular polygon;
the or each cam possesses a plurality of pairs of faces, the faces in each pair being mutually parallel, and each pair of faces being spaced apart at a distance different from the spacing between the other pairs of faces;
the cam has three pairs of faces;
each bearing element possesses a bearing plate having a groove formed in one face thereof, and at least some of the pairs of faces are dimensioned in such a manner as to be of a length in the circumferential direction of the cam that is very slightly shorter than the width of each groove;
the faces of at least one pair of faces of the cam are of a length, in the circumferential direction of the cam, which is matched to the width of the grooves in the bearing plates by means of respective setbacks provided in transitions between said faces and adjacent faces;
the or each cam is mounted to move in rotation and in translation on the or each slideway means;
the or each slideway comprises a hollow slideway-forming upright formed from one element of the implant, and a slider-forming element formed from another element of the implant, and the slideway and the slider have elongate openings passing through them to pass a cam-mounting shaft;
the cam-mounting shaft is defined by a screw passing through a central passage of the cam and through said elongate openings, and engaged in a nut;
one of the elongate openings of the slideway is a notch suitable for preventing the nut from rotating;
the bearing elements have teeth on their outside surfaces for engaging in the vertebrae lying over and under the implant; and
the implant comprises two bearing elements and a central element, two cams operating respectively between the top bearing element and the central element and between the bottom bearing element and the central element, and two slideway-forming means operating respectively between the top bearing element and the central element and between the bottom bearing element and the central element.
Other aspects, objects, and advantages of the present invention will appear better on reading the following detailed description of a preferred embodiment thereof, given by way of example and made with reference to the accompanying drawings, in which: